We have shown that nucleosome repeat lengths correlate with histone H5 levels during erythroid maturation in the adult chicken. Our proposed investigations are designed to determine whether H5 is responsible for increasing repeat length. The amount of DNA in H5-containing chromatosomes (nucleoprotein particles which contain an octamer of the four core histones plus a molecule of either H1 or H5) is longer than in H1-containing chromatosomes. Utilizing the knowledge that phosphorylated forms of H5 are found in immature but not mature erythroid cells, we propose to determine the length of DNA in H5-containing chromatosomes as a function of H5 phosphorylation to see whether modification of H5 can modulate its ability to interact with DNA. The increase in chromatin condensation and the restriction of chromatin template activity which occur during erythroid maturation can be reversed when erythrocyte nuclei are reactivated in heterokaryons formed with active tissue culture cells. Since H5 levels decrease during this process, we propose to determine whether the changes in nucleosome repeat lengths observed during maturation are reversed during nuclear reactivation in order to assess whether correlation between H5 levels and nucleosome repeat lengths is absolute. H5 will be introduced into cells via red cell-mediated microinjection. Alterations in nucleosome structure produced by the exogenous H5 will be assessed by examining chromatosomes and nucleosome repeat lengths of the microinjected cells. Gross alterations of chromatin will be assessed by inducing premature chromosome condensation in the recipient cells and by assays of cellular genetic activity.